Adhering to the Point

The
use of adhesives in stained glass restoration

Roberto Rosa

Stained
glass window of an angel by Louis Comfort Tiffany (above); and a
detail of the multi-layered construction which is typical of this
type of work (below)

A
badly damaged lamb before (above) and after epoxy edge-gluing (below)

When involved in the restoration of stained glass windows, it is of the utmost importance to retain as much
of the original fabric as possible. Modern adhesives make this task
easier.

Stained glass windows
expand and contract with variations in temperature. As they age the
lead matrix deteriorates and the window begins to deflect (buckle),
and the glass often cracks due to the stress and pressure imposed by
this cycle.

Before new and advanced
techniques were introduced to our craft, small pieces of lead were inserted
in cracked pieces of glass found during restoration. This is (still
today in some cases) good practice, as the responsible craftsman retains
the original glass, rather than replacing it with ‘a good match’. The
downside of this ‘lead repair’ is that it can, and most likely does,
interrupt the original design intent with the extraneous lead lines
used to repair the cracked glass. Also, to make room for the lead between
the two broken pieces, one of the two pieces may have to be slightly
trimmed.

Although, over the
years, the basic restoration techniques have not changed much, they
have certainly evolved, allowing the conscientious and responsible craftsman
to use these procedures for the benefit of the stained glass window
being restored. New materials, techniques and procedures have been introduced
to our craft, particularly over the past few decades, which have certainly
made a difference to the aesthetic outcome. These include: epoxy edge-gluing;
silicone edge-gluing and copper-foil repair.

It’s important to
know that these techniques differ from each other in reversibility,
strength, and visual effect, and to determine the appropriate repair
method to be used on a case-by-case basis.

EPOXY
EDGE-GLUING

Edge-gluing
broken pieces of glass with an epoxy produces a nearly invisible line.
This attribute is particularly valuable where focal points of a window
such as a face have been damaged. Furthermore, epoxy resins can be tinted
with fine pigments to match the glass, reducing the light transmission
and refraction. Epoxy can also be used for infusing microscopic cracks.

This technique produces
a very strong repair, but will most likely deteriorate in sunlight therefore
requiring a secondary glazing to protect it from UV degradation. Epoxy
is the least reversible of the three techniques and the most time consuming.

The type of epoxy
resin used is not to be confused with epoxies found on the shelves of
local hardware stores. It is specifically made for glass conservation.
Serpentino Stained
and Leaded Glass studio has been using Hxtal NYL-1 epoxy, which is an
ultra pure resin that seems to remain clear and transparent over time
and with exposure to direct light. Hxtal is supplied in two liquid parts
(A and B), both of low viscosity. It is to be weighed accurately, (one
part, by weight, of part B plus three parts, by weight, of part A).
After the two parts have been weighed into a mixing jar, they must be
mixed thoroughly. This part requires a lot of patience and must not
be rushed. As the Hxtal is mixed, numerous tiny air bubbles will be
seen to rise and pop. This is normal and to be expected, but be mindful
that the bubbles will never totally disappear. The best way to eliminate
the bubbles is with the use of a small vacuum chamber to suck the air
out of the glue. This is called ‘de-gassing’ the glue. Small vacuum
chambers are not very expensive and are quite useful.

Freshly mixed Hxtal
has a very low viscosity. If it seems too thin, let it stand (covered),
and over a period of several hours it will thicken. Thin Hxtal, however,
will penetrate cracks making them virtually disappear. The best results
are obtained when the glass is warmed to about 49°C/120°F (a hair dryer
or some other heat source can be used, taking care that the glass being
repaired is not heated too much or too quickly). Since edge-gluing of
cracked glass is performed over a light table, I turn on the light table
several hours before repairing the cracks. This will evenly warm the
glass surface of the light table, aiding the penetration of the epoxy
into the cracked glass, as the heat from the light table draws the epoxy
into the cracks.

Hxtal sets slowly
at about 24°C/75°F. It requires about one week to achieve its final
bond strength. However, ordinarily it sets after 24 hours, enough to
hold the two parts together as long as no stress is applied to the repaired
glass. Any excess epoxy should be removed with a sharp razor blade.
The use of solvents is not recommended as they may seep through the
epoxy and could have a long-term effect on the repaired piece.

SILICONE
EDGE-GLUING

Silicone has very
different properties from epoxy and provides a useful alternative for
edge-gluing. This repair technique has the lowest strength of the three
methods and should be used when a flexible joint is desirable. For instance,
in situations where the window will be under continuous stress or in
the case of plated, multi-layered windows such as ones designed and
fabricated by the American renowned artists John La Farge and Louis
Comfort Tiffany. As all components of a building expand and contract,
so do stained glass windows. In the case of plated, multi-layer windows,
as glass is stacked on top of each other, during the expansion/contraction
cycle it is best to have a repaired crack that flexes with the expansion
phase, so that it can return to its original position unaltered following
the contraction phase of the cycle.

Copper
foiling, before (above) and after repair (below)

Copper
foil being applied

One of the benefits
of using silicone to repair cracked glass is that it is easily reversible.
A sharp razor blade and some acetone will easily remove the silicone
from the edges of the glass. An important factor to be understood is
that silicone repair (edge-gluing), is not the same as smearing silicone
over the surface of the glass, thus covering the crack. Silicone is
almost as clear as epoxy, but it refracts light differently, making
it detectable at times. The same coloured pigments used with epoxy,
can be used with the silicone method reducing the light transmission
and refraction.

I particularly
like to use silicone for glass repair when working on opalescent glass
windows. A variety of small batches mixed with dyes of different colours
can be easily mixed and manipulated to match the depth and hue of the
opalescent glass and its marble-like striations and appearance for which
it is so well known. Unlike Hxtal epoxy, UV light does not affect silicone;
therefore, a secondary glazing to the exterior is not necessary. For
the last 12 years, I have been using, with excellent results, clear
silicone RTV 732 by Dow Corning. They supply it in small 2oz tubes,
which makes it easy to work with, with little or no waste.

COPPER
FOILING

The technique with
the longest history is copper foiling, and this is generally the best
option when a piece of glass has only one or two cracks. Copper foil
is a thin adhesive tape which is applied along each side of the cracked
piece, trimmed to a minimal width on the surface of the glass, and soldered.
The Tiffany Studios introduced copper foil to stained glass craft where
it was used for the manufacturing of their lamps as well as in their
stained/ opalescent glass windows, especially in and around organic,
nature scenes. The craftsmen of the Tiffany Studios mastered the use
of copper foil, producing extremely thin lines within their lamps and
windows. As restorers, we adopted their technique and material within
the restoration field. Copper foil produces a strong repair, is totally
reversible and has a negligible aesthetic impact. The final repaired
crack should have a thin line no more than 1mm in width.

All three methods
are recommended for the repair of cracked glass during restoration.
It is the craftsman’s responsibility to examine the entire project and
the relevant issues at hand before deciding which method to use on each
piece. The most important factor is that each method is reversible.
As a craftsman, I look forward to the future and for new techniques
and materials to evolve in this ‘old craft’ of ours. As long as we keep
an open mind, and are willing to learn, we will be able to restore and
preserve our beautiful stained glass windows and their history for generations
to come.